Jailbreak

Cydia

The DevTeam and others have been checking through the iPhone 4, and have noticed the baseband runs a completely different OS than on previous models. The iPhone 3G and 3G[S] both ran Nucleus OS on the PMB8878 baseband CPU (aka XGold 608), where the iPhone 2G ran the PMB8876 (aka S-Gold 2). iPhone 4 uses the PMB9800, or X-Gold 618 - running a separate OS - ThreadX, an RTOS by ExpressLogic.

UPDATE 06/29/2010 - After chatting with planetbeing today, I figured I'd add some specifics here. ThreadX is an RTOS. An RTOS is a Real Time Operating System, and in the iPhone 4's case, manages the baseband processor. This is a separate application processor than used by the main OS in the iPhone. The baseband handles the cell radio stuff - phone calls, 3G, etc. Anything using your cellular connection. Sometimes it has the GPS stuff too, although according to David Wang (aka planetbeing, from the DevTeam), it doesn't look like it does any GPS managing in this version - at least the OS treats it separately.

Says David,

...then it gets rather technical. The baseband is segregated from the main applications processor and communicates with it over i2s, spi, and uart lines. The main processor controls it with (primarily) AT commands over some other transport protocol over the SPI and UART. When you're in a call, the BBP communicates directly with the audio codec over i2s so even if the main OS is freezing up, the call will still go on uninterrupted.

The other advantage is security since if someone cracks iOS, the BBP is still separate. The two don't share any memory, so that's a security advantage.

There's no indication of why Apple moved from Nucleus over to ThreadX, although they obviously had a reason strong enough to leave what they'd been using the past few generations of their flagship device.

What's all that mean to you?

Basically, once the iPhone 4 is jailbroken, the unlock won't be an immediate release. iOS4 is already un-lockable using ultrasn0w 0.93 on the earlier iPhone models, but the payload will need to be rewritten for iPhone 4 once an exploit is discovered in the new baseband OS. Hopefully the port itself will have introduced some. No way to estimate any sort of timeline until further investigating is done.